Many types of implantable medical devices (IMDs) have been implanted that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., ventricular tachycardia or ventricular fibrillation, is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met, whereas defibrillation shocks are typically delivered when fibrillation criteria are met and an R-wave cannot be discerned from the electrogram (EGM).
Implantation of an ICD commonly uses a transvenous approach for cardiac electrodes and lead wires. The defibrillator housing or “can” is generally implanted as an active “can” electrode for defibrillation and electrodes positioned in the heart are used for pacing, sensing and detection of arrhythmias.
Patients may be asymptomatic by conventional measures but are nevertheless at risk of a future sudden death episode and are candidates for a prophylactic implantation of a defibrillator (often called primary prevention). One option proposed for this patient population is to implant a prophylactic subcutaneous implantable device (SubQ device) that does not require leads to be placed in the bloodstream. Accordingly, complications arising from leads placed in the cardiovasculature environment are eliminated. Further, endocardial lead placement may not be advised for all patients. For example some patients who have a mechanical heart valve implant or pediatric cardiac patients may be contraindicated for endocardial lead placement. For these and other reasons, a SubQ device may be preferred over an ICD for some patients.
There are technical challenges associated with the operation of a SubQ device because the SubQ device is limited to far-field sensing of cardiac signals (ECG signals) since there are no intracardiac or epicardial electrodes in a subcutaneous system. For example, SubQ device sensing is challenged by the presence of muscle noise, motion artifact, respiration and other physiological and/or non-physiological signal sources. The SubQ device needs to sense low amplitude fibrillation waves for detecting fibrillation without oversensing muscle noise or other artifact. False detection of a treatable cardiac rhythm can lead to unnecessary arrhythmia therapies, such as cardioversion/defibrillation shocks which can be painful to the patient and use considerable battery energy.
Therefore, for these and other reasons, a need exists for an improved method and apparatus to reliably sense and detect treatable arrhythmias, while rejecting noise, motion artifact, and other physiologic and/or non-physiologic signals.